Optimal operating policies for organic Rankine cycles for waste heat recovery under transient conditions

TL;DR

This paper develops optimal control policies for organic Rankine cycles in waste heat recovery for trucks, considering transient heat source fluctuations, to improve efficiency and reduce losses under realistic driving conditions.

Contribution

It introduces dynamic optimization methods to determine optimal operating policies for ORC systems under transient conditions, including scenarios with power limitations.

Findings

Operating at minimal superheat is generally optimal.

Flexible superheat policies can significantly reduce losses during power limitations.

Extended power restrictions may lead to bypassing exhaust gases, reducing efficiency.

Abstract

Waste heat recovery for trucks via organic Rankine cycle is a promising technology to reduce fuel consumption and emissions. As the vehicles are operated in street traffic, the heat source is subject to strong fluctuations. Consequently, such disturbances have to be considered to enable safe and efficient operation. Herein, we find optimal operating policies for several representative scenarios by means of dynamic optimization and discuss the implications on control strategy design. First, we optimize operation of a typical driving cycle with data from a test rig. Results indicate that operating the cycle at minimal superheat is an appropriate operating policy. Second, we consider a scenario where the permissible expander power is temporarily limited, which is realistic in street traffic. In this case, an operating policy with flexible superheat can reduce the losses associated with operation at minimal superheat by up to 53 % in the considered scenario. As the duration of power limitation increases, other constraints might become active which results in part of the exhaust gas being bypassed, hence reduced savings.